Current controlled recall schema

ABSTRACT

A memory circuit includes a controlled current source coupled to an input to a nonvolatile cell, and a second controlled current source coupled to a volatile cell, the volatile cell coupled to receive current from the controlled current source via the nonvolatile cell.

PRIORITY CLAIM

The present application claims priority to the U.S. provisional patentapplication titled RECALL SCHEMA, having application No. 60/919,372,filed on Thursday, Mar. 22, 2007, which is incorporated herein byreference.

TECHNICAL FIELD

The present disclosure relates to memory circuits.

BACKGROUND

In high density memory circuits, very low recall currents and otherfactors can sometimes lead to false, premature latching by the volatilememory cell. Techniques to boost the recall currents are one solution,but false latching may still occur when the larger recall currents causeimbalanced potentials in the volatile latch.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, the same reference numbers and acronyms identifyelements or acts with the same or similar functionality for ease ofunderstanding and convenience. To easily identify the discussion of anyparticular element or act, the most significant digit or digits in areference number refer to the figure number in which that element isfirst introduced.

FIG. 1 illustrates a memory cell comprising an SRAM cell coupled to anon-volatile memory cell.

FIGS. 2 and 3 are illustrations of embodiments of a current-controlledRECALL scheme.

FIG. 4 is a timing diagram of an embodiment of signaling to controlcurrent during RECALL.

DETAILED DESCRIPTION

References to “one embodiment” or “an embodiment” do not necessarilyrefer to the same embodiment, although they may.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” Words using the singular or pluralnumber also include the plural or singular number respectively.Additionally, the words “herein,” “above,” “below” and words of similarimport, when used in this application, refer to this application as awhole and not to any particular portions of this application. When theclaims use the word “or” in reference to a list of two or more items,that word covers all of the following interpretations of the word: anyof the items in the list, all of the items in the list and anycombination of the items in the list.

“Logic” refers to signals and/or information that may be applied toinfluence the operation of a device. Software, hardware, and firmwareare examples of logic. Hardware logic may be embodied in circuits. Ingeneral, logic may comprise combinations of software, hardware, and/orfirmware.

Those skilled in the art will appreciate that logic may be distributedthroughout one or more devices, and/or may be comprised of combinationsof instructions in memory, processing capability, circuits, and so on.Therefore, in the interest of clarity and correctness logic may notalways be distinctly illustrated in drawings of devices and systems,although it is inherently present therein.

FIG. 1 illustrates a memory cell comprising an SRAM cell 100 coupled toa non-volatile memory cell 120, hereinafter referred to as an nvSRAMcell 101. An nvSRAM memory device typically includes a plurality ofnvSRAM cells that are integrated with a controller onto a singlesemiconductor chip to form an array.

Generally, an nvSRAM cell 101 comprises a plurality of n-channel,Field-Effect Transistors (FETs); a plurality of nonvolatile elements,such as nonvolatile transistors (SONOS) or capacitors; and a pluralityof resistors. It should, however, be appreciated that other types oftransistors, such as p-channel FETs, and combinations of different typesof transistors can be utilized.

The nvSRAM cell 101 is comprised of a static random access memory (SRAM)cell 100 that is capable of communicating a bit of data to and from anexterior environment and a non-volatile (nv) cell 120 for providingnonvolatile backup storage to the SRAM cell 100. The SRAM cell 100 iscapable, as long as power is being provided, of receiving a bit of datafrom an exterior environment, retaining the bit of data, andtransmitting the bit of data back to the exterior environment. If,however, power is removed from the SRAM cell 100, the SRAM cell 100 willlose the bit of data. The nv cell 120 prevents loss of the bit of databy providing the capability to receive the bit of data from the SRAM100, retain the bit of data in the absence of power being provided tothe SRAM cell 100, and return the bit of data to the SRAM cell 100 whenpower is restored. For example, the bit of data can be transferred fromthe SRAM cell 100 to the nv cell 120 in a STORE operation. At a latertime, the bit of data can be returned from the nv cell 120 to the SRAMcell 100 in a RECALL operation. The SRAM cell 100 can then transmit thebit of data to the exterior environment, if desired.

Typically, the nvSRAM cell 101 is combined with other nvSRAM cells toform a memory array that is integrated onto a single semiconductor chip.Typical memory array embodiments comprise 1 Mbyte or 4 Mbytes of nvSRAMcells. One motivation for creating integrated semiconductor chips withever larger arrays of nvSRAM cells is that the area per cell decreasesas more cells are integrated onto a chip. For example, four chips, eachwith arrays of 262,144 nvSRAM cells (an array of 512×512), occupy agreater surface area than a single chip with an array of 1,048,576nvSRAM cells (1 Mbyte, an array of 1024×1024). The area occupied by amemory chip is important because many applications for the chip, such aspersonal computers, are continually attempting to provide more memory inless space.

In the illustrated embodiment, the volatile portion 100 is a StaticRandom Access Memory (SRAM) cell. Four transistors of the cell areshown. Typical embodiments include two other “pass” transistors (notshown here) to couple and decouple the internally stored bit informationto external interfaces (such as bit lines). The word ‘static’ indicatesthat the memory retains its contents as long as power remains applied.Random access means that locations in the memory can be written to orread from in any order, regardless of the memory location that wasaccessed last.

Each bit in the SRAM cell 100 is stored, along with its complementvalue, on four transistors that form two cross-coupled inverters. TheSRAM cell has two stable states, which are used to denote a logical 0and a logical 1. Two additional transistors (not shown) may serve tocontrol external access to the SRAM bits during READ and WRITEoperations. The first cross coupled inverter formed by transistors 112,114 and the second cross coupled inverter formed by transistors 113, 115reinforce each other to retain charges representing a volatile bit atnode DT (the ‘true’ node) and the complement of that volatile bit atnode DC (the ‘complement’ node).

The output of the first inverter DT is coupled to the input of thesecond inverter and the output of the second inverter DC is coupled tothe input of the first inverter. The configuration of the first andsecond inverters form a latch in the SRAM cell 100. The ‘bulk’ region ofthe p-type transistors 112, 113 may be tied to Vcc (logical high), andthe bulk region of the n-type transistors 114, 115 may be tied to Vss(e.g. logical low or ground).

The nonvolatile cell 120 includes a first silicon oxide nitride oxidesemiconductor (SONOS) FET 106 and a second SONOS FET 107 forrespectively backing up the ‘true’ and ‘complement’ bits of the SRAMcell 100. The state of SRAM cell 100 is stored by the first and secondSONOS FETs 106, 107 by performing a STORE operation. This may beaccomplished by applying a high (higher than logical 1) voltage at nodeSE (the gates of SONOS FETs 106, 107). It should be appreciated that theSONOS FETs 106, 107 can be replaced with other types of nonvolatilestorage elements, such as silicon nitride oxide semiconductor (SNOS)transistors, floating gate transistors, and capacitors, to name a few.

The nonvolatile cell 120 further includes a transistors 108, 109 thatoperate as switches during STORE and RECALL operations to appropriatelyconfigure the nonvolatile cell 120 for the transfer of a bit of databetween the volatile cell 100 and the nonvolatile cell 120. Morespecifically, the transistors 108, 109 function to connect thenonvolatile cell 120 to the volatile cell 100 during STORE and RECALLoperations and to otherwise disconnect the two cells. The state of thetransistors 108, 109 is controlled by signal at node STR. Morespecifically, if STR is low, the transistors 108, 109 are turned off todisconnect the nonvolatile cell 120 from the volatile cell 100.Conversely, if STR signal is high, the transistors 108, 109 are on toconnect the nonvolatile cell 120 to the volatile cell 100. Thenonvolatile cell 120 further includes transistors 104, 105 that functionas switches to place the nonvolatile cell 120 in the appropriateconfiguration for transferring a bit of data from the nonvolatile cell120 to the volatile cell 100 during RECALL operations. Morespecifically, the transistors 104, 105 function to connect thenonvolatile cell 120 to node VCCT during a RECALL operation and todisconnect the nonvolatile cell 120 from VCCT otherwise. The transistors104, 105 are typically turned off during a STORE operation. The state ofthe transistors 104, 105 is controlled by a signal that is applied tonode RC. More specifically, if RC signal is low, the transistors 104,105 are turned off, to disconnect the nonvolatile cell 120 from VCCT.Conversely, if RC is high, the transistors 104, 105 are turned on toconnect the nonvolatile cell 120 to VCCT.

A RECALL operation may begin by clearing the existing SRAM data, forexample by discharging external bit lines (not shown), clamping nodeVCCI to node VSSI, and then coupling nodes DT and DC to the external bitlines. In this manner the nodes DT and DC, which store the volatilestate, may be discharged to ground (e.g. Vss). The nodes DT and DC arethen decoupled from the external bit lines. Both nodes STR and RC aredriven high, providing a charging path to DT and DC through thenonvolatile cell 120 from the power supply node VCCT. With SE held lowand assuming the last STORE operation left SONOS transistor 106 in an‘erased’ state (representing storage of a logical 1) and SONOStransistor 103 in a ‘programmed’ state (representing storage of alogical 0), and assuming that the erase threshold is −1 V and theprogram threshold is +1V, the SONOS transistor 106 will conduct currentwhile the SONOS transistor 103 will not because its gate voltage isbelow its threshold voltage. Node DT charges up to logical high, whilenode DC remains at logical low, thereby reestablishing in the volatilecell 100 the data corresponding to the last STORE cycle. The RECALLoperation may be completed by powering up the SRAM and returning allcontrol lines to their default states.

One potential problem during RECALL is that the volatile cell 100 maylatch prematurely, capturing a state that does not represent the stateof the nonvolatile cell 120. Many factors may contribute topre-latching, among them a weak current flow through the ‘erased’nonvolatile charge storage transistor (due for example to drift in thetransistor's threshold voltages and other factors), leakage currentthrough the ‘programmed’ nonvolatile charge storage, and, mainly,transistor mismatch. The transistors of the SRAM bitcell are notdesigned to follow the rules of a highly accurate analog amplifier. Theyhave a large mismatch due to channel length variation and thresholdvoltage variation. Conventional recall schemas react to the mismatch ofnearly all transistors of the nvSRAM bitcell. The approach describedherein “switches” six of these transistors electrically off duringrecall, making it more efficient.

Pre-latching occurs when combined effects act to cause the voltages atVCCI and VSSI to pre-maturely separate by too great an amount, beforethe actual state of the nonvolatile cell 120 is re-enforced in thevolatile cell 100. This may occur when imbalances in the volatile cell100 temporarily overpower the driving effects of the nonvolatile cell120.

To alleviate these problems somewhat, the RECALL process may be modifiedin some embodiments, as follows. The volatile cell 100 is discharged asdescribed above, driving DT and DC at or close to ground level. VCCI isclamped to VSSI at a voltage level near ground. The voltage at SE isdriven down to a negative value, for example −1V, to help ensure thetransistors 102 and 103 are both off. Next, the RECALL from thenonvolatile cell 120 is activated, by turning RC and STR on. The voltageat SE is ramped from a negative to a positive value, for example from−1V to 1V. Current will now flow thru one of the transistors 106, 107that is in an erased state (storing a logical 1). The SRAM node suppliedby the transistor that is on, either DT or DC, will charge up to avoltage comparable to VCCT (less the voltage drop across the nonvolatilecell 120). VCCI is unclamped from VSSI and the SRAM cell is recharged.

The above modification in the RECALL process may act to reducepre-latching by reducing the effects of leakage current from the‘programmed’ transistor and by making the current flow from the ‘erased’transistor more robust. This may result in the nonvolatile cell 120driving the volatile cell 100 more strongly toward a properrepresentative state, reducing the likelihood of pre-latching.

However, another problem may arise if the above modifications cause aRECALL current into the volatile cell 100 that is too strong. Aspreviously discussed, VCCI and VSSI may be clamped together(electrically connected to conduct current between them) in order toalleviate voltage imbalances between the two nodes that can lead topre-latching. However, the clamping is typically performed with atransistor (such as an enhancement mode CMOS transistor) which has someimpedance. This impedance in the clamp transistor may cause anincreasing voltage differential between VCCI and VSSI as the currentflowing through the clamping transistor, Icmp, increases. If thisvoltage differential becomes too great, pre-clamping may occur. Thecurrent Icmp flowing through the clamping transistor is typicallyproportional to the current flowing into the volatile cell 100 from thenonvolatile cell 120 during RECALL.

FIGS. 2 and 3 are illustrations of embodiments of a current-controlledRECALL scheme. A current regulating circuit 200 is coupled to VCCI andVSSI in order to alleviate conditions that can lead to pre-latching.Furthermore, a controlled current source 301 may be provided in place ofa voltage source at VCCT, increasing the robustness of the currentdriven from the nonvolatile cell 120 to the volatile cell 100 duringRECALL.

The current regulator 200 may comprise transistors 201, 202, and 203.Transistor 203 is controlled by a gate voltage Vssi_gate, and may beoperated during RECALL to clamp VSSI close to ground (Vss) potential.Transistor 202 is controlled by a gate voltage Vcmp, and may be operatedduring RECALL to clamp VCCI close to the potential at VSSI. Thus, duringRECALL, VCCI and VSSI may be held close to one another in potential,until the state of the nonvolatile cell 120 is represented in thevolatile cell 100 (causing a small differential between VSSI and VCCI),at which point the clamping may be removed, the volatile cell 100 isrecharged, and VCCI and VSSI float to a wider and more stabledifferential reflecting the state of the nonvolatile cell 120.

Transistor 201, which is controlled by a gate voltage Vcci_gate, may actto further regulate the voltage differential and hence the current Icmpbetween VCCI and VSSI during RECALL. In one embodiment, transistor 201is a p-type CMOS transistor. Transistor 201 may be operated as a weakpull up device to further control the current between VCCI and VSSI.

With the use of transistor 201, ramping of the voltage at SE duringRECALL may be reduced or eliminated. Ramping SE from a negative to apositive voltage has the effect of gradually turning on the storagetransistor having an erased state. This urges the potentials at the twonodes VCCI and VSSI of the volatile cell 100 to start to separate in thecorrect direction. Transistor 201 may be operated to limit the currentthrough the volatile cell 100, which in affect controls the separationrate of VCCI and VSSI and also affects the speed at which the RECALL maybe completed by successfully representing the state of the nonvolatilecell 120 in the volatile cell 100. SE may instead be set to a voltagelevel high enough to ensure the charge storage transistor that stores a‘1’ is turned on, and vice versa (or at least turned on more than thetransistor storing a ‘0’). The nodes VCCI and VSSI may start to separatewithout ramping SE, because the volatile cell 100 floats up or down tothe relative threshold of SE. This makes for a more robust recall, whichis easier to control.

The current to the volatile cell 100 from the nonvolatile cell 120 mayalso be controlled. A controlled current source 301 at VCCT may providea controlled source of current during RECALL.

The rate at which the signals STR and RC are changed from their lowestto highest values (and/or vice versa) may be controlled in order toprevent sudden spikes in current demand on the current source 301 whenlarge numbers of cells are RECALLED simultaneously.

FIG. 4 is a timing diagram of an embodiment of signaling to controlcurrent during RECALL. VCCI is held at approximately Vcc by thedepletion mode transistor 201 (Vcci_gate low, t<T0). VSSI is held atapproximately Vss by the enhancement mode transistor 203 (Vssi_gatehigh, t<T0). VCCI and VSSI are clamped and pulled down to a value closeto Vss, and the volatile cell is discharged (T0<t<T2. Vcci_gate andVssi_gate are set to values that keep the voltage potential low betweenVCCI and VSSI (T0<t<T4). The nonvolatile cell 120 is turned on (RC high,SE low positive voltage, STR high, T1<t<T5) and charging of DT and DCnodes from current from the nonvolatile cell 120 begins. The nonvolatilestate is latched in the volatile cell 100, VCCI and VSSI are unclamped,and the nonvolatile cell is turned off (RC, SE, and STR brought low,t>T5).

In one embodiment, the voltage SE at the gates of the nonvolatile chargestorage transistors 106, 107 may be dynamically adjusted during RECALLto reduce current detected flowing through the current regulator 200.This current to gate voltage feedback loop may have the effect offurther stabilizing the RECALL process.

Those having skill in the art will appreciate that there are variousvehicles by which processes and/or systems described herein can beeffected (e.g., hardware, software, and/or firmware), and that thepreferred vehicle will vary with the context in which the processes aredeployed. For example, if an implementer determines that speed andaccuracy are paramount, the implementer may opt for a hardware and/orfirmware vehicle; alternatively, if flexibility is paramount, theimplementer may opt for a solely software implementation; or, yet againalternatively, the implementer may opt for some combination of hardware,software, and/or firmware. Hence, there are several possible vehicles bywhich the processes described herein may be effected, none of which isinherently superior to the other in that any vehicle to be utilized is achoice dependent upon the context in which the vehicle will be deployedand the specific concerns (e.g., speed, flexibility, or predictability)of the implementer, any of which may vary. Those skilled in the art willrecognize that optical aspects of implementations may involveoptically-oriented hardware, software, and or firmware.

The foregoing detailed description has set forth various embodiments ofthe devices and/or processes via the use of block diagrams, flowcharts,and/or examples. Insofar as such block diagrams, flowcharts, and/orexamples contain one or more functions and/or operations, it will beunderstood as notorious by those within the art that each functionand/or operation within such block diagrams, flowcharts, or examples canbe implemented, individually and/or collectively, by a wide range ofhardware, software, firmware, or virtually any combination thereof.Several portions of the subject matter described herein may beimplemented via Application Specific Integrated Circuits (ASICs), FieldProgrammable Gate Arrays (FPGAs), digital signal processors (DSPs), orother integrated formats. However, those skilled in the art willrecognize that some aspects of the embodiments disclosed herein, inwhole or in part, can be equivalently implemented in standard integratedcircuits, as one or more computer programs running on one or morecomputers (e.g., as one or more programs running on one or more computersystems), as one or more programs running on one or more processors(e.g., as one or more programs running on one or more microprocessors),as firmware, or as virtually any combination thereof, and that designingthe circuitry and/or writing the code for the software and/or firmwarewould be well within the skill of one of skill in the art in light ofthis disclosure. In addition, those skilled in the art will appreciatethat the mechanisms of the subject matter described herein are capableof being distributed as a program product in a variety of forms, andthat an illustrative embodiment of the subject matter described hereinapplies equally regardless of the particular type of signal bearingmedia used to actually carry out the distribution. Examples of a signalbearing media include, but are not limited to, the following: recordabletype media such as floppy disks, hard disk drives, CD ROMs, digitaltape, and computer memory; and transmission type media such as digitaland analog communication links using TDM or IP based communication links(e.g., packet links).

In a general sense, those skilled in the art will recognize that thevarious aspects described herein which can be implemented, individuallyand/or collectively, by a wide range of hardware, software, firmware, orany combination thereof can be viewed as being composed of various typesof “electrical circuitry.” Consequently, as used herein “electricalcircuitry” includes, but is not limited to, electrical circuitry havingat least one discrete electrical circuit, electrical circuitry having atleast one integrated circuit, electrical circuitry having at least oneapplication specific integrated circuit, electrical circuitry forming ageneral purpose computing device configured by a computer program (e.g.,a general purpose computer configured by a computer program which atleast partially carries out processes and/or devices described herein,or a microprocessor configured by a computer program which at leastpartially carries out processes and/or devices described herein),electrical circuitry forming a memory device (e.g., forms of randomaccess memory), and/or electrical circuitry forming a communicationsdevice (e.g., a modem, communications switch, or optical-electricalequipment).

Those skilled in the art will recognize that it is common within the artto describe devices and/or processes in the fashion set forth herein,and thereafter use standard engineering practices to integrate suchdescribed devices and/or processes into larger systems. That is, atleast a portion of the devices and/or processes described herein can beintegrated into a network processing system via a reasonable amount ofexperimentation.

The foregoing described aspects depict different components containedwithin, or connected with, different other components. It is to beunderstood that such depicted architectures are merely exemplary, andthat in fact many other architectures can be implemented which achievethe same functionality. In a conceptual sense, any arrangement ofcomponents to achieve the same functionality is effectively “associated”such that the desired functionality is achieved. Hence, any twocomponents herein combined to achieve a particular functionality can beseen as “associated with” each other such that the desired functionalityis achieved, irrespective of architectures or intermedial components.Likewise, any two components so associated can also be viewed as being“operably connected”, or “operably coupled”, to each other to achievethe desired functionality.

1. A memory circuit comprising: a controlled current source coupled toan input to a nonvolatile cell; and a second controlled current sourcecoupled to a volatile cell, the volatile cell coupled to receive currentfrom the second controlled current source via the nonvolatile cell. 2.The memory circuit of claim 1, wherein the second controlled currentsource coupled to a volatile cell further comprises: the second currentsource coupled to reduce potentials between at least two nodes of alatch of the volatile cell.
 3. The memory circuit of claim 1, whereinthe second controlled current source coupled to a volatile cell furthercomprises: at least one pull up transistor and at least one pull downtransistor.
 4. A RECALL process to cause the state of a nonvolatilememory cell to be represented in a volatile memory cell comprising:applying a current regulator to prevent premature latching of bit valuesby a volatile memory cell during a RECALL operation from a nonvolatilememory cell to the volatile memory cell.
 5. The RECALL process to causethe state of a nonvolatile memory cell to be represented in a volatilememory cell of claim 4, wherein applying a current regulator to preventpremature latching of bit values by a volatile memory cell furthercomprises: applying the current regulator to balance a potential betweentwo nodes of a latch of the volatile memory cell.
 6. The RECALL processto cause the state of a nonvolatile memory cell to be represented in avolatile memory cell of claim 5, wherein applying the current regulatorto balance a potential between two nodes of a latch of the volatilememory cell further comprises: operating a pull up transistor coupled toa first one of the two nodes, and operating a pull down transistorcoupled to a second one of the two nodes.
 7. The RECALL process to causethe state of a nonvolatile memory cell to be represented in a volatilememory cell of claim 4, further comprising: applying a controlledcurrent source to an input of the nonvolatile memory cell during theRECALL.
 8. The RECALL process to cause the state of a nonvolatile memorycell to be represented in a volatile memory cell of claim 4, furthercomprising: adjusting a voltage at a gate of a nonvolatile chargestorage transistor on the nonvolatile memory cell, the voltage at thegate adjusted to reduce a current detected flowing in the currentregulator.